The present invention pertains to the field of aircraft environmental control systems, and is more particularly concerned with providing an improved method and apparatus for optimizing engine bleed air extraction while improving passenger comfort. Environmental control systems are commonly used on both commercial and military aircraft transports, as well as smaller commercial aircraft. The most applicable use of the present invention is for commercial aircraft that have electronic control capability, and varying passenger loading. The invention is also applicable to aircraft with a less sophisticated control system which desire improved fuel economy and passenger comfort.
Briefly stated, an environmental control system (ECS) provides conditioned, pressurized air to cool (or heat) ventilate and pressurize an aircraft cabin. The ECS typically incorporates a flow control valve whose purpose is to regulate the inflow of fresh air into the cabin to a predetermined level. This level is determined by the fresh air ventilation requirements of the cabin occupants and the amount of cooling capacity required to achieve a comfortable cabin environment. On earlier aircraft utilizing low-pressure water separation systems, the temperature of the fresh air could not be lower than 32.degree. F. to avoid ice formation in the water removal devices and the aircraft ducting. This generally provided more fresh air flow for cooling purposes than was actually required for ventilation. However, with the introduction of high pressure water separation, the fresh air supplied by the air conditioning system could be provided at subfreezing temperatures, with recirculated cabin air raising the temperature to above freezing before entering the main aircraft distribution system. The cooling requirements, therefore, could be met with a smaller amount of fresh air than for a comparable low pressure water separation system. This type of system is detailed within U.S. Pat. No. 4,198,830, herein incorporated by reference.
Current systems optimize the relationship between fresh air flow and recirculation flow to balance the fresh air requirements with the cooling requirements and the need for circulation of air through the cabin. The total amount of airflow required to maintain adequate circulation is primarily a function of the cabin volume. Modern aircraft having sophisticated electronic systems can reduce the amount of fresh air flow into the cabin as a function of a reduced passenger loading. By inputting the number of passengers on board, an electronic flow control valve can be adjusted to provide only the fresh air required for those passengers. This has the benefit of saving fuel while still providing a comfortable cabin environment. However, when reducing the fresh air flow, the total ventilation rate is also reduced. This could result in less than optimum air circulation throughout the aircraft cabin, creating "dead" zones of warm, cold or stale air. Some aircraft attempt to counteract this occurrence to some degree by selective activation of recirculation fans, but these are typically combined with low pressure water separation systems that provide more fresh air flow than is required for cooling. Thus, in order to obtain an optimum total ventilation rate, more fans would be required than is actually necessary to provide for normal operation. The associated extra weight and power required for these fans is disadvantageous within the limitations inherent within aircraft applications.
The present invention is intended to maintain the optimum cabin air circulation, and hence passenger comfort, while achieving the fuel savings associated with reductions in fresh air flow with passenger loadings lower than the design point of the aircraft.